A One Percent Chance

A One Percent Chance

Every year at this time, I bring attention to a tree disease that rewrote the book on urban forestry.

Dutch Elm Disease (DED) is a fungal disease that, after decimating the elm forests of Europe early in the last century, was discovered in New York in the 1930s and has since crossed the continent. It is described as 99 percent fatal to Ulmus Americana, the American elm tree.

Rapid growth and the ability to survive tough urban conditions are two of the characteristics that made American elms the go-to tree for urban settings. Graceful, vase-like shapes lent themselves well to over-street canopies and spectacular alleys.

The only major drawback was the tree’s inability to survive the introduced fungal pathogen Ophistoma ulmi. This disease, combined with the presence of a previously harmless bark beetle, created the perfect conditions for a country-wide epidemic.

DED moved at a rapid pace, with the fungus’s sticky spores clinging to the bodies of both native and introduced elm bark beetles. This, combined with people moving beetles and diseased firewood, spread DED throughout most of the native range of this stately, street tree.

With the trees having such a high mortality rate, there were many who predicted the eventual extinction of the American elm. However, there were—right from the beginning—tiny numbers of trees that survived, in spite of being in the middle of epidemic numbers of dead and dying trees and being plagued by hordes of bark beetles. These trees continued to grow and prosper.

A number of programs funded and promoted by the governments of Canada and the United States have brought forward many potentially resistant examples of American elm. Through rigorous testing, some are proving to be very resistant to DED.

American efforts have resulted in American elm varieties like “Princeton,” “Valley Forge,” and “ Delaware,” all of which have been shown to have some resistance to DED.

Canadian efforts to date have resulted in several trees of note, including “ Brandon,” an American elm, and “Jacan,” a Japanese elm pioneered by Dr. Wilbur Ronald, formerly from Agriculture Canada’s Morden, Manitoba, facility.

This later selection is in fact growing in Brandon, Manitoba, on 10th Street. And on Pacific Avenue at 15th Street, “Jacan” does not have the elm’s characteristic vase-like shape, but it has proven to be an excellent tree in other respects.

As we move forward in an age of cloning and micropropagation, efforts in Guelph, Ontario, spearheaded by Dr. Praveen Saxena have resulted in the ability to mass produce resistant trees. This greatly increases the efforts to grow and test resistant varieties as well as producing them for introduction.

With so many skilled and dedicated people working to find resistant American elms, one can’t help but have hope that from the small numbers of survivors a new generation of elms will emerge and take their places on our city streets.

Brad Gurr is an ISA-certified arborist and plant health care specialist dedicated to the care and preservation of trees. For more information on this subject or any plant health care related questions, please feel free to contact him online at: bgurr